The present invention relates to a process for the production of diisocyanates of the diphenylmethane series with very high contents of 2,4′-methylenediphenyl diisocyanate, and to a process for the production of prepolymers and polymers from these diisocyanates.
Aromatic isocyanates are important raw materials for the production of polyurethane materials. In this connection the diisocyanates and polyisocyanates of the diphenylmethane series (MDI) play the greatest role, quantitatively.
Polyisocyanates of the diphenylmethane series are understood to denote isocyanates and isocyanate mixtures of the following type:
where n denotes a natural number >2.
Similarly, polyamines of the diphenylmethane series are understood to denote compounds and compound mixtures of the following type:
where n denotes a natural number >2.
It is known that diisocyanates and polyisocyanates of the diphenylmethane series (MDI) are produced by phosgenation of the corresponding diamines and polyamines of the diphenylmethane series (MDA). The diamines and polyamines of the diphenylmethane series (MDA) are themselves produced by condensation of aniline and formaldehyde. The corresponding diisocyanates, 2,2′-MDI, 2,4′-MDI and 4,4′-MDI, which are described in the specialist circles as 2-ring (i.e. bi-nuclear) compounds of MDI (i.e. diisocyanates of the diphenylmethane series), are obtained by phosgenation of diamines of the diphenylmethane series. During the condensation of aniline and formaldehyde, the 2-ring i.e. (bi-nuclear) MDA (methylenediphenyldiamine), however, also continues to react further with formaldehyde and aniline to form higher-nuclear (i.e. poly-nuclear or poly-ring) MDA types, which after the phosgenation constitute the polynuclear content in the polymeric MDI (i.e. polyisocyanates of the diphenylmethane series).
The crude MDI mixture produced in the phosgenation can be separated in the polymer/monomer separation by means of simple evaporation or distillation into 2-nuclear-MDI (i.e. monomeric MDI) and a polymer-MDI fraction (i.e. polymeric MDI or PMDI). The isomer mixture of the 2-nuclear-MDI fraction contains, in addition to the diisocyanates 2,2′-MDI, 2,4′-MDI and 4,4′-MDI, some secondary components such as solvent residues or phenylisocyanate derivatives. The monomeric 2-nuclear-MDI fraction is separated, according to the prior art, by distillation or by crystallisation into the 4,4′-MDI isomers and into a mixture comprising about 50% 2,4′-MDI and 50% 4,4′-MDI. The two monomeric products are then supplied as polyurethane raw material to the world market, or they are processed further with polymeric MDI into mixed products.
At the present time, very pure 2,4′-MDI is not available commercially in large-scale amounts. This has not changed despite the fact that many positive properties of 2,4′-MDI have recently become known. Thus, in polyurethane flexible foam systems, for example, a 2,4′-MDI can replace the conventional TDI system of 2,4-TDI and 2,6-TDI (as described in EP-B1 0676434). Also, 2,4′-MDI can be successfully used in heat-curable one-component polyurethane systems (as described in EP-B1 0431331).
The 4,4′-isomer of MDI is the most important MDI isomer on account of its high reactivity and its extremely good ability to form hard segments. By comparison, the 2,4′-isomer, on account of its different reactivity, is characterised, in particular, by use in very low viscosity, relatively low monomer content prepolymers.
Thus, WO-A 93/09158 describes prepolymers with low monomer contents. By reacting isocyanates and polyethers containing secondary hydroxyl groups, at NCO/OH ratios of 1.6 to 1.8, and when using variously reactive NCO groups in the isocyanate molecule, results in low monomer content prepolymers. Monomeric MDI with a content of 92 wt. % of 2,4′-diisocyanatodiphenylmethane is given as an example of a suitable isocyanate. Important areas of use are adhesives and coatings, and, in particular, film composite systems with low migration values.
One application of diphenylmethane diisocyanates is in the production of film composites, which are employed in the foodstuffs packaging sector. The hygienically satisfactory, cheap packaging of foodstuffs guaranteeing a high storage stability is nowadays achieved with composite films. These composite films are formed by bonding films with different barrier properties such that the composite can optimally be adjusted to the respective requirements.
Polyurethanes represent the adhesives of choice. These adhesives are used in solvent-containing and solvent-free form. Due to the trend towards solvent-free systems, two-component systems consisting of a polyol mixture and a prepolymer based on isocyanates have become increasingly popular. Due to the traces of moisture adsorbed on the film surfaces, it has proved necessary to employ adhesives with a relatively large excess of isocyanate groups compared to OH groups.
This large excess of isocyanates also constitutes, however, a limiting factor in the further processing of the film composites, since the film composites must be free of aromatic amines before they can come into contact with foodstuffs.
Therefore, before the composite films can be processed further, they must be stored until amines are no longer detectable. This time period depends on many factors, such as, for example, the properties of the employed adhesive, the nature of the films (e.g. type and thickness), and the prevailing temperature and atmospheric humidity. In this regard, the presence of aromatic amines in the test foodstuffs can presumably be explained by the fact that monomeric isocyanates that have not fully reacted migrate through the thin films and slowly react with moisture on the surface to form polyureas, which are stable under normal storage conditions for foodstuffs. Until this reaction has gone to completion, any monomeric isocyanates that are present can also be partially hydrolysed to amines by the test foodstuffs used in the test.
Due to the much greater reactivity of the NCO group located in the 4-position compared to that of the NCO group located in the 2-position in MDI, MDI isomers with at least one NCO group located in the 4-position can be integrated significantly faster into an oligomeric or polymeric polyurethane network, and can thus be prevented from migrating through the film. Accordingly, the concentration of still free, unbound monomeric MDI isomers with NCO groups located in the 4-position (i.e. 4,4′-MDI and 2,4′-MDI) falls rapidly after production of an MDI prepolymer, and in the processing of this MDI prepolymer with polyols. Due to its significantly lower reactivity, the 2,2′-MDI on the other hand remains longer in monomeric form in the adhesive layer than the other MDI isomers, and can therefore, take longer to migrate through the film. Accordingly, the content of 2,2′-MDI in the MDI isomer mixture is a critical quantity and should be as low as possible.
The following information is also known from the prior art regarding the production of MDI.
The production of MDI mixed products that contain the various MDI isomers, by a specific synthesis of the MDA containing the corresponding MDA isomer, is known and described in the literature. EP-B1 158059 discloses the production of a particularly high 2-nuclear content in an MDA mixture containing ca. 80% 4,4′-MDA and ca. 10% 2,4′-MDA with a 2-nuclues content of ca. 90%. On the other hand, the yield of 2,4′-MDA can be purposefully increased, as is disclosed in EP-B1 3303 with an MDA which contains 88 wt. % of 2-nuclear MDA with 19 wt. % of 2,2′-MDA, 36 wt. % of 2,4′-MDA and 45 wt. % of 4,4′-MDA. In particular, the production of monomer-rich MDA types containing a large content of 2,4′-MDA generally leads to a large amount of 2,2′-MDA as by-product. On account of its lack of reactivity, the 2,2′-MDI formed from the resulting 2,2′-MDA is, however, undesirable in large concentrations in many applications.
The production of high polymer content MDA with 2-nuclear contents of from 46% to 65% is described in, for example, DE-A1 2750975 and DE-A1 2517301.
The essential parameters by means of which the proportion of 2,4′-MDA can be adjusted in the condensation of aniline and formaldehyde are known. As a rule, the content of 2-nuclear MDA is adjusted by the excess of aniline in the condensation. The proportion of 2,4′-MDA present in the 2-nuclear MDA can be adjusted by a low degree of protonation during the condensation, or in other words, by a low molar ratio of HCl: aniline such as, for example, <0.2:1, or by a high reaction temperature, as described in DE-A1 3407494.
The large-scale production of isocyanates by reacting the corresponding amines with phosgene in solvents is known and is described in detail in the literature (Ullmanns Enzyklopädie der technischen Chemie, 4th Edition, Vol. 13, pp. 347-357, Verlag Chemie GmbH, Weinheim, 1977). The MDA phosgenation leads first of all to a crude MDI mixture. Also, the production of monomeric MDI and polymeric MDI from the crude MDI mixture by distillation or crystallisation is, in principle, known in the relevant literature.
Basically, two main products are isolated according to the prior art from the crude monomeric 2-nuclear MDI fraction of the originally crude MDI mixture. The first of the two main products of the isomer separation is a 4,4′-MDI-rich isomer mixture (“4,4′-product”), which is practically free from 2,2′-MDI and which also contains <3 wt. % of 2,4′-MDI. The second of the two main products of the isomer separation is a 2,4′-MDI-rich mixture (“2,4′/4,4′-product”), which contains from 20 to 70 wt. % of 2,4′-MDI and up to 3 wt. % of 2,2′-MDI, with the remainder being 4,4′-MDI. To produce these two main products, the following two industrial processes starting from the crude monomeric 2-nuclear MDI fraction that was obtained from the crude MDI mixture from the polymer/monomer separation are, as a rule, used at the present time:    a) distillation, as described in, for example, DE-A1 3145010 and/or DE-A1 2631168;or    b) crystallisation, as described in, for example, EP-A2 482490 and/or DE-A 2532722.
Specialist in the polyurethane and polyisocyanate fields are presently concentrating on the most economical production process of the monomeric isomer mixtures, of the “4,4′-product” and of the “2,4′/4,4′-product” (M. Stepanski, P. Faessler: “New hybrid process for purification and separation of MDI isomers”, Sulzer Chemtech, Presentation at the Polyurethane Conference 2002 in Salt Lake City, October 2002).
The production of relatively highly concentrated 2,4′-MDI starting from an MDA mixture with high contents of 2,4′-MDA that was obtained by condensation of aniline and formaldehyde, with a low degree of aniline protonation, is described in, for example, WO-A1 02/070581. A purification involving the removal of 2,2′-MDI from the 2,4′-MDI that is obtained is not envisaged in the process according to WO-A1 02/070581. However, particularly in the production of 2,4′-rich MDA mixtures with a low degree of protonation, i.e. with a low ratio of HCl to aniline, disproportionately large amounts of 2,2′-MDA are formed. This is described in, for example, EP-B1 3303. These large amounts of 2,2′-MDA or 2,2′-MDI, respectively have to be removed at least partially after phosgenation and before the mixtures are used in the polyurethane production. In fact, however, the purity of the 2,4′-MDI with respect to quantity of 2,2′-MDI present is an essential quality feature that is even more important than the purity with respect to 4,4′-MDI.
The process described in WO-A1 02/070581, in which an MDI mixture with a high content of 2,4′-MDI is produced by phosgenation of a corresponding MDA mixture with a high content of 2,4′-MDA, corresponds to the conventional method described in the prior art for producing an MDI mixture containing ca. 50 wt. % of 2,4′-MDI and ca. 50 wt. % of 4,4′-MDI. Since 2,4′-MDI is a low boiling point compound compared to 4,4′-MDI, 2,4′-MDI is obtained as overhead product by distillation. The significant factor in this case is that further low boiling point compounds, and in particular 2,2′-MDI, accumulate in the overhead product. If 4,4′-MDI is separated from the 2,4′-fraction by conventional means, then there is obtained as a first product the first main product “4,4′-MDI product” already mentioned above, that generally contains about 1-2 wt. % of 2,4′-MDI, and in addition, as a second main product, the second main product “2,4′/4,4′-MDI product” also already mentioned above, which forms a mixture of 2,4′-MDI and 4,4′-MDI in the vicinity of the eutectic point. In this connection, the 2,2′-isomer on account of its boiling point accumulates in the eutectic mixture. A typical content of 2,2′-MDI in this eutectic mixture is between 0.8 and 5 wt. %.
A similar situation exists in the separation of “4,4′-MDI product” by crystallisation. In this case the 2,2′-isomer is necessarily concentrated with the 2,4′-rich fraction in the mother liquor. If the resultant mother liquor is now separated into the isomers 2,4′-MDI as distillate and 4,4′-MDI in the bottom of a distillation column, the undesired and unreactive 2,2′-MDI likewise accumulates in the desired 2,4′-MDI fraction. Thus, depending on the initial quality of the crude 2-nuclear MDI fraction, a 2,4′-MDI containing 0.8 to 5 wt. % of 2,2′-MDI is obtained. Furthemore, low boiling point secondary components such as phenyl isocyanates and traces of solvents can pass into the distillate, if these are not initially removed from the starting mixture.
If, on the other hand, an attempt is made to separate the resultant mother liquor containing the isomers 2,4′-MDI and 4,4′-MDI by crystallisation, then first of all the eutectic point must be exceeded using a non-crystallisation process in order not to obtain only pure 4,4′-MDI crystallisate and a 2,4′-containing mother liquor with high contents of 4,4′-MDI. For this reason, no highly enriched 2,4′-MDI can be obtained by a pure crystallisation process starting from the crude 2-nuclear MDI fraction. The eutectic point may be exceeded, for example, by a distillation process as described above. In this case, the process disclosed in the present patent application for the production of very pure 2,4′-MDI fractions can alternatively be carried out in step d) using a crystallisation process for separating the major proportion of 4,4′-MDI.
In DE-A 2631168, a process is described for the production of MDI mixtures with low contents of residual chlorine using distillation methods. According to the process described in DE-A-26 31 168, MDI mixtures containing 2,4′-MDI in an amount of more than 97 wt. % and having less than 50 ppm of chlorine can also be produced in this way.
The large content of 2,2′-MDI interferes in practically all end-use applications and areas since it is unreactive and can be completely incorporated into a polymer network only under drastic reaction conditions. In most cases, significant amounts of 2,2′-MDI remain as residual monomer in the processing and are possibly released over time, or they react in a random manner with atmospheric moisture and lead to impaired polymer properties. Also, residual traces of solvents are undesirable in the polyurethane production and these adversely affect the product quality due to their unpleasant smell. Traces of phenyl isocyanates that are possibly contained in turn act as chain terminators in the polyurethane reaction and also adversely affect the polymer properties.